The present invention relates to an improved x-ray tube, particularly to same of the x-ray transmission variety.
X-ray tubes are well-known in the art, a particular type thereof, namely, the x-ray transmission target tubes, being widely used. Because in the transmission tube, electrons strike a window that also serves as the tube anode, and produce the x-rays, the resulting x-rays must be able to penetrate the window of the tube to reach the outside world. Therefore, such a window generally must be relatively thin to minimize the x-ray absorption therein.
As a result of the requirement of a thin window in these tubes, they are, of necessity, power-limited since their operation at higher power levels results in rapid and excessive heating of the window, leading to damage thereto and eventual failure of the tube. Because transmission target tubes are necessarily operated at lower power levels, e.g., about 2 watts, most of the x-ray tubes that are employed are of the back-reflection type, which produce only relatively narrow x-ray beams, i.e., beams having a beam divergence angle .theta., of less than 40.degree. and generally beams having a .theta. value of about 15.degree. to 30.degree..
As compared to transmission target x-ray tubes, the achievement of an equivalent x-ray output from a reflection type tube requires greater power input than is so for transmission target tubes, this being so up to a certain power level, e.g., about 2 watts, to which the transmission target tubes are, as a practical matter, limited. Consequently, due to their power limiting factor, a substantially reduced x-ray output results from the use of transmission target tubes which do, however, provide a soft x-ray beam quality that is superior for many applications. A glass window tube may be used but this results in an undesirable degree of x-ray attenuation from the tube.
Where a wider angle beam (i.e., .theta. value of about 40.degree.) is desired and/or where a soft x-ray beam is sought, it is generally necessary to utilize a back-reflection x-ray tube, with a large beryllium window, which is not always desirable from a mechanical and reliability standpoint.
One proposed solution to the thermal problem in transmission x-ray tubes operated at higher power levels, is the provision of a solid heat sink on the transmission window of the tube. However, this is of limited benefit and usefulness since the maximum power level at which such tubes can be operated is on the order of about 2 Watts.
The present invention provides an x-ray transmission target tube that has significant advantages over prior art devices and affords other benefits as well.